Bicyclists and motorists frequently display a this-road’s-not-big-enough-for-the-both-of-us attitude, and it’s usually all bluster.

But on Monday afternoon, an altercation between a bicyclist and a motorist ended with cracked skull and a felony assault charge.

The dispute began as a 42-year-old bicyclist (wearing no helmet) pedaled up Lake Merced Boulevard and reprimanded a 36-year-old driver for not sharing the road, according to Officer Hermann Chu of the Taraval Police Station. Insults were exchanged as the two men headed north, side-by-side, through a green light at Winston Drive. That’s when a witness saw the car driver reach through his window and push the bicyclist, who fell hard on the asphalt and lost consciousness.

At San Francisco General Hospital, doctors determined that the bicyclist had a fractured skull and collar bone. Although the motorist claimed he never touched the bicyclist, police arrested him on felony assault charges.